If you are hiking the Inca trail, definitely bring an emergency rain poncho. We hiked during the "dry season" and on day 3 it poured. My backpack and everything in it would've been soaked without the poncho. A light rain jacket to put over a thin fleece is perfect for hiking during the day so you can peel the layers off and stuff them in your daypack. You go through 4 seasons in a day, so you need everything! Extra, non-sweaty socks for the campsite are also nice. So are earplugs if the people in the tent next to you (or the other person in your tent) snore. It gets very cold at night, so make sure your bag is rated for freezing conditions or rent one from the travel company and bring a liner so you can minimize contact with the well-used bag.
Finally, definitely, definitely buy the walking stick at Ollantaytambo. I thought it would be unnecessary and figured I could ditch it if it proved a hassle to carry, but that thing saved me! (Especially going downhill). Really. Bring it.
Toiletries and Medical Supplies: Biodegradable body wipes are a great idea for cleaning off after hiking all day.
Mosquito repellent with DEET. You only have to deal with the mosquitos for a little while since they are not around at the very high altitudes, but that little while is enough to earn you misery for the rest of your trip if you don't keep them at bay.
Photo Equipment: Extra camera batteries are a must!
Luggage and bags:
Pack your own backpack light, water proof it for camera and other electronics
Clothing/Shoes/Weather Gear: Bring a change of dry clothes. Chances are you'll get wet during the day. It helps a lot to put on dry clothes when you get to camp.
Toiletries and Medical Supplies: It would help to bring bio-degradable mouth wash. Tooth brushing is a hassle with no running water and also pollutes the environment.
Photo Equipment: Bring extra batteries for digital camera. Nowhere to charge it on the trail.
Camping/Beach/Outdoor Gear: It's very cold at night. Bring a warm hat and mittens.
Miscellaneous: The hiking company provide snacks. Do bring your personal favorite snacks. They can be purchased in Cusco or Ollantaytambo.
Metallic walking sticks are forbidden in the Inca Trail. But you may buy these bambu sticks at the beginning in Ollantaytambo or even at the Trail entrance gate. Don't pay more than 3 soles (1 USD) for them.
I had never used walking sticks, but they happened to be useful, specially in slippery grounds and when climbing down steps.
Luggage and bags:
Note that they do not allow large back packs into the park. You have to check them in at the entrance to the park.
Clothing/Shoes/Weather Gear: Most definitly you should have a water proof jacket as the rain is very unpredictable
Photo Equipment: A wide angle lense if you own a SLR would be great pics
Miscellaneous: A hiking pole would be ideal if you have weak knees. There is a lot of climbing and walking down.
However they do have hiking poles at the gate that you can rent I did not enquire.
Luggage and bags: I brought my shoes from home, but there are plenty from which to choose at the sporting good shops in Cusco. These shops specialize in hiking gear and the staff are knowledgeable and helpful - English is usually not a problem. The only caveat - and this applies to home as well - is to make sure you have some time to "break in" your boots. If you are hiking the Inca Trail, these boots will save your life. The Incas were expert masons - everything revolved around well placed stone work (the stones are often so tight - completely without mortar - that you can't even squeeze a butter knife in between them!). Many parts of the trails are laid with stone which actually makes the hiking more difficult than if you were to trek over dirt paths. You'll want to be sure your "boots are made for walking!"